This invention relates to low pressure fuel supply systems for supplying fuel which tends partially to solidify at low ambient temperatures and whose viscosity increases with decreasing temperature, from a fuel tank to an injection pump.
Well known examples of such fuels are middle distillate fuels such as diesel fuels. When such fuels are subjected to low temperatures, their higher molecular weight hydrocarbon components tend to precipitate out of the liquid phase as wax crystals. In a fuel supply system these crystals can then after a short time block fuel filters, resulting in fuel starvation. The temperature of a fuel below which waxing occurs is termed the cloud point of the fuel, although filter blocking may only occur at temperatures substantially below the cloud point.
Diesel fuels for use in different geographical regions are tailored to alleviate the problem of waxing. It is necessary to design a fuel to provide optimum fuel viscosity at the expected operating temperature of a fuel supply system, which is affected by the ambient temperature. Fuels are therefore designed with higher content of higher molecular weight hydrocarbon components for use in hotter regions, so that adequate fuel viscosity is maintained at higher fuel supply system temperatures. Such fuels then have higher cloud points however. Lower contents of higher molecular weight hydrocarbon components are required in cooler regions to prevent excessive fuel viscosity at low temperatures and to reduce waxing. Abnormally cold ambient temperatures in any given region can still however lead to waxing if the temperature falls significantly below the cloud point of the fuel used in that region.
It is known to use thermostatically controlled valves or heaters to control diesel fuel temperatures to prevent waxing. Disadvantages arise however because fuel supply systems for use in different regions having different ambient temperatures, or in vehicles travelling from one region to another, require different thermostat settings for each region. If this is not done, since fuel viscosity at any given temperature varies according to the design of the fuel, the fuel viscosity maintained by a thermostat varies with the type of fuel used. Fuel viscosity variation then leads to reduced performance due to changes in injection pump backleak pressure causing changes in fuel supply timing and fuel delivery, and changes in the internal leakage in the pump giving variation in the volume of fuel delivered.